Methods and devices provided herein relate to a new imaging mode for scanning probe microscopy in a liquid environment. The specific design and methodology, for example, provides atomic force microscopy (AFM) imaging of a material covered in liquid, where the AFM probe, when dynamically operated, experiences a significantly lowered damping and has a corresponding increase in Q factor when compared to conventional systems, thereby achieving high sensitivity and high resolution dynamic imaging of samples in liquid.
Conventional AFM systems, when imaging material in liquid, submerse the cantilever portion of the AFM probe in the liquid. The relatively large footprint of the cantilever (typically having an area of about 40 μm by 300 μm) in liquid (such as water) is associated with high hydrodynamic damping and low quality factor when it is driven to resonant oscillation. Although such systems may satisfactorily operate in air (due to the small viscosity of air, the associated hydrodynamic damping experienced by the AFM probe is small), they are not ideal for imaging material submersed in liquid. The significant hydrodynamic drag experienced by the AFM probe fully submersed in liquid results in low image resolution. This disadvantage is even more pronounced when imaging soft materials in liquid, where the tapping mode operation (e.g., the AFM probe in dynamic oscillation) and the use of small AFM tip-sample interaction forces are required to avoid unwanted deformation or damage to the soft material. Large hydrodynamic damping, low Q factor, high thermal noise, and large tip-sample interaction related to the use of an AFM probe operated in the dynamic AFM imaging mode all contribute to the problem of obtaining accurate and high resolution information about the sample material being imaged.
These drawbacks become particularly relevant in certain applications, such as the imaging of living biologic materials, including cultured biological cells, characterized as “soft” (e.g., eukaryotic animal cells). To remain viable, such a material must be immersed in suitable liquid (e.g., culture media), and cannot be subject to large contact forces from an AFM tip. The requirements for liquid media and low contact forces, however, directly impact the ability of AFM to achieve high resolution imaging. Accordingly, there is a need in the art for scanning probe microscopy systems capable of high resolution imaging of materials immersed in liquid, including soft materials where the contact force between the scanning probe and the material must be sufficiently small to avoid unwanted material deformation or damage.